With the paint films used on automobiles the development of paints which have excellent scratch resistance and durability has proceeded rapidly in view of the problem of scratch damage which arises with car-wash machines for example. In the past softening the paint film itself by reducing the Tg and crosslink density of the paint film and improving the scratch resistance has been considered as a means of resolution which improves the scratch resistance but, although the car-wash damage resistance is improved, this results in the other features such as hardness, staining resistance, acid resistance and the like which are required of the paint film obtained being lost.
Paint compositions which contain as essential components (A) acrylic copolymers of acid value from 25 to 125 and hydroxyl group value from 30 to 150 which contain (meth)acrylic acid derivative units, (B) acrylic copolymer of epoxy equivalent from 230 to 1500 and hydroxyl group value from 30 to 150 which contain unsaturated monomer units which have epoxy groups, where the (A) and (B) components are copolymers which may include structural units based on unsaturated monomers which are ε-caprolactone modifications of acrylic monomers which have hydroxyl groups, and (C) amino resin are known as a method of obtaining paint films which have excellent acid resistance and scratch resistance (for example, see Japanese Unexamined Patent Application Laid Open H5-171103). However, with these paint compositions there is a weakness in that the acid rain resistance is reduced since they have an amino resin as an essential component.
Furthermore, similarly high solid fraction paint compositions which are characterized by the fact that they contain in the proportions indicated (A) from 5 to 30 wt % of hydroxyl group containing lactone modified oligomer of weight average molecular weight not more than 1000 and of which the hydroxyl group value is from 200 to 800, (B) from 5 to 50 wt % of hydroxyl group containing resin of weight average molecular weight from 1,000 to 6,000 and of which the hydroxyl group value is from 50 to 200, (C) from 30 to 70 wt % of polyisocyanate compound and (D) from 3 to 30 wt % of melamine resin are known as paint compositions in which two types of hydroxyl group containing resin are used and which have excellent finished appearance, paint film hardness, acid rain resistance and scratch resistance, and which have a high solid fraction (for example, see Japanese Unexamined Patent Application Laid Open 2002-105397). However, with these paint compositions the glass transition point (Tg) of the paint film is inevitably reduced by the use of the low molecular weight hydroxyl group containing oligomer and there is a weakness in that the staining resistance and the weather resistance are reduced.
Furthermore, paint compositions which have as essential components (a) from 30 to 90 parts by weight of a lactone modified acrylic polyol resin which has been obtained by the ring-opening addition reaction in the absence of a catalyst on adding from 10 to 200 parts by weight of lactone compound to 100 parts by weight of an acrylic polyol resin of which the hydroxyl group value is from 75 to 250 mgKOH/g and the acid value is from 0.5 to 50 mgKOH/g, (b) from 10 to 70 parts by weight of polyisocyanate compound and (c) from 0.01 to 20 parts by weight of a specified alkoxysilane partially hydrolyzed condensate are known as paint compositions where the staining resistance and impact resistance are excellent and where at the same time the appearance, weather resistance, water resistance and the like are also excellent (for example, see Japanese Unexamined Patent Application Laid Open 2003-313493). However, with these paint compositions there is a weakness in that the balance of the paint film performance in terms of the car-wash damage resistance and the acid rain resistance, staining resistance and weather resistance cannot be maintained satisfactorily because of the presence of one type of lactone modifier acrylic polyol resin.